Browse evidence-based analysis of health-related claims and assertions
None of the studies said, 'Eat less fat to live longer' — in fact, some warned it might not help or could even be harmful.
Descriptive
The diets weren’t just about eating less fat — they also banned butter, added lots of vegetable oil, and cut out eggs, so we don’t know which part actually mattered.
The studies weren’t perfectly done — many didn’t hide who was on which diet, and people had to remember what they ate, which might have messed up the results.
All the studies were done on men who already had heart problems — we don’t know if the same results would apply to women or people without heart disease.
One study that tried to get people to eat only 10% saturated fat actually saw more people die than in the group that ate more fat.
Quantitative
The diets tested in the studies weren’t even the same as the official government advice — they were different and less strict.
Even though people who ate less fat had much lower cholesterol, they didn’t live longer or have fewer heart attacks than those who didn’t change their diet.
Before the government told everyone to eat less fat, no study had actually tested whether eating exactly that much fat helped people live longer.
Eating less saturated fat didn’t help men who already had a heart attack live longer without another heart attack.
In men who already had heart problems, eating less fat didn’t help them live longer, even though their cholesterol went down.
The studies used in this review mostly involved men, so we can't be sure the results apply to everyone, like women or different ethnic groups.
For people who haven't had a heart attack yet, how much fat they eat doesn't seem to affect their chance of dying from heart disease, according to combined data from several long-term studies.
The official advice to eat less fat to avoid dying of heart disease isn't backed up by the long-term studies of large groups of people.
Eating more saturated fat — like in butter or red meat — doesn't appear to raise the chance of dying from heart disease, based on long-term studies of nearly 90,000 people.
Correlational
Eating more fat in your diet doesn't seem to make you more likely to die from heart disease, according to long-term studies of nearly 90,000 people.
This review looked at data from over 347,000 adults followed for an average of 14 years — so if saturated fat had even a moderate effect on heart disease, this study would have been able to find it.
Even when researchers accounted for how many total calories people ate or how much other fats they consumed, the link between saturated fat and heart disease still didn’t show up.
The studies in this review didn’t all agree with each other, but the reason for the disagreement wasn’t because of differences in how old the people were, how long they were followed, or how good the studies were.
One study looked at whether eating more unsaturated fat compared to saturated fat helps the heart — and found no link, even though other research has suggested it might.
Studies that found a link between saturated fat and heart disease may be more likely to get published than those that found no link, which could make the overall picture look more uncertain than it really is.
Different studies used different ways to measure what people ate — some asked about food habits over a year, others just asked what they ate yesterday — which might make it harder to see real links between diet and heart disease.
We don’t have enough reliable data yet to say if swapping butter for bread or olive oil makes a difference in heart disease risk, because not enough studies provided the right information.
Whether you're young or old, male or female, eating more saturated fat doesn't seem to affect your heart disease risk any differently, according to this big review.
Overall, eating more saturated fat doesn't seem to increase your risk of having any kind of heart disease or stroke, according to a large review of 21 long-term studies.